1. Field of Invention
The present disclosure generally relates to disk drives and more particularly to a method and apparatus for detecting defects in a hard drive assembly.
2. Description of Related Art
Disk drives are magnetic recording devices used for the storage of information. The information is typically recorded on concentric tracks on either surface of one or more magnetic recording disks. To facilitate the storage and retrieval of data in an orderly manner, disks are typically organized in blocks called sectors. These sectors are located on the disk by a set of unique identifiers called cylinder (or track), head and sector numbers. The disks are rotatably mounted to a spin motor and information is accessed by means of read/write heads that are mounted to actuator arms. These actuator arms are maneuvered via voice coil motors, wherein the voice coil motor is excited with a current to rotate the actuator and move the heads.
The movement of the actuator is controlled by a servo system. One well known type of servo system is referred to as a dedicated servo, wherein one side of the disk is dedicated to servo information. By using this servo information, the actual radial positions of the heads can be determined, and after comparison with desired head position information, control signals can be sent to the actuator arm to adjust the head position accordingly.
The servo system typically sends control signals to the actuator in accordance with a position error signal (PES) derived from the servo information. Typically, the PES has a magnitude indicative of the relative distance between the head and the center of a track and a polarity indicative of the direction of the head with respect to the track center. Additionally, the PES generally is generated by the servo system by comparing the relative signal strengths of burst signals on the disk surface. It is common during read/write operations to compare the absolute value of each PES sample to a predetermined safe-threshold value in order to verify the correct positioning of the head. It is generally known in the art that there are two situations which may give rise to a PES value which exceeds a safe-threshold value. First, it is possible that the head is misaligned to such an extent that its distance from the center of the track is sufficient to exceed this safe-threshold value. Second, it is likewise possible that a defect exists in the servo information, resulting in a misreported PES value, despite the fact that the head is positioned correctly. This latter scenario is what has led to the practice of using high PES values as an indication that there may be a defect in the servo burst area.
As is generally known in the prior art, using a high PES value to detect servo defects is a cumbersome process. This is due to the fact that the sector with the highest PES value often is not the sector which actually contains the defect. Thus, it is generally considered necessary to check several of the sectors surrounding the high PES sector before labeling any given sector as defective. Moreover, this conventional method for defect detection may also result in mapping continuous sectors as servo defects where a high PES is actually due to an error which occurred during the servo track writing process.
Accordingly, there is a need for an improved approach to detecting servo defects on a disk in a disk drive assembly, such approach avoiding the cumbersome process of taking several PES samples around a defective sector to pinpoint the defective sector.
The present disclosure comprises an apparatus, method and computer program product for a hard disk drive defect detection system. In one embodiment, a method of detecting potential defects on the surface of a disk having a plurality of tracks is recited. The method comprises reading the servo bits for a first sector and a second sector, determining a burst measure for the first sector, said burst measure being a function of the burst signals provided by the servo bit of the first sector. This method further comprises determining a reference value that is a function of the burst signals of the servo bits for the first sector and second sectors, comparing the burst measure of the first sector to the reference value, and designating said first sector as abnormal where the burst measure of said first sector differs from the reference value by more than a predetermined amount. In one embodiment, the predetermined amount is between 5 percent and 50 percent.